He wants to watch me with another girl

So my partner said it would be okay if I got it on with my girl friend but he would want to watch.. Which I guess is only fair if im being pleasured by someone else - but I can't seem to get my head around him seeing another girl naked and in the act. I know this sounds selfish but I'm very intrested in having sex with this girl but don't know what I should do about my partner being involved ?